1. Field of the Invention
This invention relates to the production of tubes of .beta. alumina ceramic.
.beta. ALUMINA CERAMIC IS A MATERIAL HAVING A NOMINAL COMPOSITION BY WEIGHT OF 5% Na.sub.2 O, 95% Al.sub.2 O.sub.3. The amount of sodium oxide in practice can range from 5 up to 10%. Up to 5% of magnesium oxide or lithium oxide may also be included in addition to the sodium oxide. The present invention is concerned more particularly with the production of long thin walled tubes of this material. Such tubes are required for use as a solid electrolyte in certain types of cells, e.g. sodium sulphur cells and for the electrochemical recovery of sodium from solution in other metals. The tubes must be impervious to helium and have a density which closely approaches the theoretical single crystal value. The tubes for use as an electrolyte must also have the composition closely controlled right up to the surface of the material.
2. Description of the prior art
In making a tube of .beta. alumina ceramic by conventional firing processes, sintering temperatures in the range of 1600.degree. to 1900.degree.C are required but they would have to be very closely controlled, for example to .+-. 10.degree.C, over the whole length of the tube to obtain optimum properties. Such fine control for long tubes would normally require a very slow heating stage. In the course of heating up to the sintering temperature the material therefore spends long periods of time at lower temperatures. Under these conditions processes occur which degrade its surface energy in such a way that it is not available for sintering when the required temperature has been reached. This results in production of material of moderate density even when very fine starting materials are used. Furthermore the material contains a volatile constituent (Na.sub.2 O) which can be lost during an extended firing, leading to a loss of control of the overall composition and to the existence of composition gradients within a given material.
It is well known that the rate processes which cause grain growth and sintering in ceramic materials increase exponentially with temperature. For a .beta. alumina ceramic tube for use as an electrolyte in a sodium sulphur cell, it has been found that detrimental changes in crystalline structure occur using a conventional sintering process as described above with sintering in the range of 1600.degree.-1900.degree.C. In U.S. Pat. Nos. 3,468,719 and 3,475,225 of Tennenhouse, it has therefore been proposed to effect the sintering at a temperature between the minimum sintering temperature and an upper temperature of 1550.degree.C (or of 1650.degree.C in the presence of conversion inhibiting amounts of extraneous sodium ion). The sintering time is prolonged (2-18 hours depending on the temperature).